The present invention relates to a nuclear magnetic resonance imaging (MRI) method and to apparatus for continuously measuring a nuclear magnetic resonance (hereinafter this is referred to as xe2x80x9cNMRxe2x80x9d) signal obtained from hydrogen or phosphorus in an object to be examined, and for imaging the density distribution and relaxation time distribution or the like of a nuclear particle in the object.
As a receiving coil to detect an NMR signal generated from an object in an MRI apparatus, a high sensitivity coil called a xe2x80x9cmultiple RF coilxe2x80x9d or xe2x80x9cphased array coilxe2x80x9d has been used in recent years (Japanese Patent laid open No. 2-500175). A multiple RF coil is a coil dedicated to signal reception composed of an array of small type RF coils having a relatively high sensitivity, which is capable of receiving a signal in a wide field of view with high sensitivity. Various types of multiple RF coils have been proposed, including a static magnetic field type or a detecting part. In operation, the signals received with the respective unit coils of this multiple RF coil are combined to produce an image of the object to be imaged.
On the other hand, a method to shorten the imaging time by thinning out data in the phase encoding direction and using multiple coils has been proposed, for example, by [4] Daniel K Sodickson and Warren J Manning in an article entitled xe2x80x9cSimultaneous obtainment of spatial harmonics(SMASH): fast imaging with radio frequency coil arraysxe2x80x9d, in Magnetic Resonance in Medicine, Vol. 38, pages 591-603, (1997), and by [5] J.Wang and A.Reykowski in an article entitled xe2x80x9cA SMASH/SENSE related method using ratios of array coil profilesxe2x80x9d, in ISMRM 99. This kind of technology is referred to as a space encoding method or parallel MRI. An aliasing artifact in thinning phase encoded data is removed using the spatial difference between respective sensitivity distributions of a multiple RF coil. However, for the removal of this aliasing artifact, a highly accurate calculation using a highly accurate sensitivity distribution of the RF coil is necessary. This operation is performed in a measurement space (k space) in the method described in the above literature [4]. And, in the method described in the literature [5], the operation is performed in real space after Fourier transformation.
Generally, a sensitivity distribution of the multiple RF coil can be calculated from each of the received RF signals. More specifically, a method that images a phantom having a uniform density previously and regards the spatial shading of an image as a sensitivity distribution of the multiple RF coil, and a method that calculates it by use of a low frequency filter in connection with an image measured separately from the object, are known. However, these traditional methods involve the following problems.
That is, in the traditional methods, the process of calculating the sensitivity distribution of said RF coil is performed before imaging. Thus, for example, when imaging a part of an object, such as the abdomen of a patient, which changes its shape temporally with breathing, the shape of the object examined is likely to be different between the time the sensitivity distribution is calculated and the time the imaging is performed.
Also, the spatial arrangement of the RF coil fitted to the object is liable to change in accordance with the movement. In addition, in case of imaging a patient during a surgical operation, the position of the RF coil can change with time.
Also, in case there is a demand to display these images in real time, the traditional method of using sensitivity data of the coils obtained previously can lead to an error, with a result that the quality of the image can be deteriorated.
Furthermore, measuring the sensitivity distribution of an RF coil prior to imaging extends the total imaging time, and so it creates a problem in that the effect of short-time imaging, which is a main characteristic of this technology, is deteriorated.
The present invention has been developed for the purpose of solving the above-mentioned problems.
To solve the above-mentioned problems, the present invention provides a magnetic resonance imaging method which involves the steps of:
(a) applying an RF pulse, a slice encoded gradient magnetic field, a phase encoded gradient magnetic fields, and a readout gradient magnetic field to the object to be examined, that is placed in a uniform static magnetic field, in accordance with a predetermined pulse sequence, and executing this pulse sequence repeatedly;
(b) detecting an NMR signal generated from the object to be examined by executing said step at each small type: RF coil composing a multiple coil, and storing the NMR signals in connection with the k space separately;
(c) performing a process of calculation of the sensitivity distribution of each coil using data only of a low spatial frequency region (hereinafter this is referred to as a xe2x80x9clow regionxe2x80x9d) at each k space; and
(d) performing the image composing process using the sensitivity distribution of a coil calculated in said step (c) and the measured data stored in connection with said k space.
The magnetic resonance imaging method of the present invention also involves steps of:
(a) preparing a k space having a predetermined matrix size for which to memorize NMR signals detected from the object to be examined;
(b) executing a pulse sequence for NMR imaging of the object to be examined, which has been placed in a uniform static magnetic field;
(c) memorizing NMR signals obtained by executing said pulse sequence into said k space;
(d) calculating the sensitivity distribution of a plural number of small type signal receiving coils forming a multiple coil by using one part of the NMR signal for imaging, which is memorized in connection with said k space; and
(e) composing the image by using the sensitivity distribution of the small type signal receiving coil calculated above and the NMR signal memorized in connection with said k space.
The magnetic resonance imaging method of the present invention is a method of imaging an object to be examined by using a multiple coil without the presence of an aliasing artifact, and involves the steps of:
(a) preparing a k space having predetermined matrix for which size to memorize NMR signals detected from the object to be examined with a number corresponding to small type signal receiving coil composing a multiple coil;
(b) executing a pulse sequence for the NMR imaging of the object to be examined, which is placed in uniform static magnetic field; the data in low region in the phase encoded direction being measured fine in the k space, and the data in high spatial frequency region (hereinafter referred to as a xe2x80x9chigh regionxe2x80x9d) being measured roughly when this pulse sequence is executed;
(c) memorizing the measured data for imaging obtained by executing said pulse sequence in connection with the k space corresponding to a small type coil;
(d) calculating the sensitivity distribution of a plural number of small type signal receiving coils forming a multiple coil by using a part of the data measured for imaging and memorized in connection with said k space; and
(e) composing an image of all fields of view of the multiple coil by using said calculated sensitivity distribution of said small type signal receiving coil and measured data memorized in connection with said k space.
The magnetic resonance imaging method of the present invention is a method for continuously imaging an object to be examined by using a multiple coil, comprising:
(a) preparing a k space having a predetermined matrix size to memorize NMR signal detected from the object to be examined;
(b) executing a pulse sequence for imaging a first NMR image of the object to be examined, which has been placed in a uniform static magnetic field;
(c) memorizing NMR signals obtained by executing said pulse sequence in connection with said k space;
(d) calculating the sensitivity distribution of a plural number of small type signal receiving coil which form a multiple coil by using a part of the NMR signals for imaging and memorizing it in connection with said k space;
(e) composing an image by using said calculated sensitivity distribution of a small type signal receiving coil and said NMR signal memorized in connection with said k space; and
(f) executing a pulse sequence for imaging after the second image and composing an image by applying said memorized sensitivity distribution of said small type signal receiving coil to the obtained NMR signal.
Problems addressed by the present invention also can be solved by provision of a magnetic resonance imaging apparatus. In this regard, a magnetic resonance imaging apparatus of the present invention comprises:
a magnet for generating a uniform static magnetic field within the space accommodating the object to be examined;
a multiple coil comprised of a plural number of small type coils for detecting NMR signals generated from said object to be examined, said plural number of small type coils being arrayed to overlap a part of adjacent coils;
means for applying a high frequency magnetic field, a slice encoded gradient magnetic field, a phase encoded gradient magnetic field and a readout gradient magnetic field to image said object to be examined, where the phase encoding direction is directed in the direction of arrangement of said multiple coil;
means for controlling said magnetic field applying means, including means for modifying a step change in the application amount of said phase encoded gradient magnetic field in its high region relative to its low region;
measured data memorizing means for memorizing NMR signals detected in said multiple coil corresponding to each small type coil;
means for calculating the sensitivity distribution of each small type coil by using data of a low region in said phase encoded direction in every NMR signal detected by each small coil; and
means for composing an image from said sensitivity distribution and said data memorized in said measured data memorizing means.
The magnetic resonance imaging apparatus of the present invention comprises:
a magnet for generating uniform static magnetic field within a space accommodating the object to be examined;
a multiple coil comprised of a plural number of small type coils, which are arranged to overlap with each other in a part of the adjacent coils, to detect NMR signals generated from said object to be examined;
means for applying a high frequency magnetic field, a slice encoded gradient magnetic field, a phase encoded magnetic field and a readout gradient magnetic field in accordance with a predetermined pulse sequence to image said object to be examined;
a k space for memorizing NMR signals detected by said multiple coil corresponding to each small type coil;
means for controlling said magnetic field applying means, including means for modifying a step change in the high region relative to the low region of the k space memorized measured data;
means for calculating the sensitivity distribution of each small type coil by using the data of the low region in said k space in every NMR signal detected by a small type coil; and
means for composing an image from the sensitivity distribution and the data memorized in said memorizing means.
The magnetic resonance imaging apparatus of the present invention comprises:
means for executing a measurement repeatedly for imaging a predetermined slice of an object to be examined in an imaging unit;
means for calculating the sensitivity distribution of a multiple coil by using at least one part of measured data obtained for one image and memorizing it;
means for composing an image of all fields of view of the multiple coil by using said calculated sensitivity distribution; and
means for applying the memorized sensitivity distribution without renewal to said image composition based on measured data after calculating the sensitivity distribution.
According to the present invention, because the sensitivity distribution of a small type RF coil is calculated from data obtained in a measurement for imaging, there is no difference in time between generation of the sensitivity distribution data and the measured data. Thus, even in MRI measurement in which the condition changes from time to time, it is easy to maintain stability. And, even in a measurement that demands real-time measurement, an error is not invited and the image quality is not deteriorated.
And, according to the present invention, since filtering is performed so as to connect the data of low region to zero smoothly in calculating the sensitivity distribution using data of the low region in the k space, a sensitivity distribution with no influence of an aliasing artifact can be calculated.
Also, since the sensitivity distribution of the RF coil is measured prior to imaging using measured data for imaging, without measuring the sensitivity distribution of the RF coil, the total imaging time does not have to be extended, and the effect of imaging in a short time does not to be degraded.
According to the present invention, in an embodiment for measuring an image continuously, the collection of measured data and image reconstruction following it are performed continuously, and control to display a plural number of images that last in terms of time can be performed. Also, the image reconstruction means uses the sensitivity distribution calculated from one measured data for reconstructing a plural number of images. Thus, in dynamic imaging in parallel MRI, the time for image reconstruction can be shortened and real-time measurement can be improved.